Previously we proposed interpolated averaged CT (IACT) as a low dose alternate of cine average CT (CACT) for improved PET/CT attenuation correction (AC) in oncology and cardiology applications. This study aims to evaluate the effectiveness of both average CT protocols in myocardial perfusion SPECT/CT. We simulated a patient injected with Tc-99m-sestamibi using the digital 4D Extended Cardiac Torso (XCAT) phantom. We modeled the respiratory motion of 2, 3 and 4 cm with period of 5.9 s. The average attenuation and activity maps represented CACT and static SPECT, while the attenuation maps of end-inspiration and end-expiration represented 2 helical CTs (HCT-in and HCT-ex). The IACT was obtained by averaging the HCT-in, HCT-ex and the interpolated phases between them generated by B-spline method. Sixty noise-free and realistic noisy projections with attenuation modeling were generated from RAO to LPO positions using an analytical projector to model a low energy high resolution parallel-hole collimator. Data were reconstructed with different AC maps (CACT, HCTs and IACT) using OS-EM method with up to 200 updates. Polar plots were generated from the short-axis images and 17-segments were drawn on the plots. Relative difference (RD) of the average intensity was computed for each segment using the original phantom as the reference. Slight artifacts can be observed in the reconstructed images and polar plots using the HCTs AC. For motion amplitude = 2 cm, the maximum RD for CACT, IACT, HCT-in and HCT-ex were 0.7%, 2.3%, 6.1% and 8.0% respectively. The RD of the HCTs notably deviated from the phantom in the basal inferolateral and anterolateral regions especially for higher motion amplitudes. Our findings are consistent with our former results of cardiac PET/CT. We conclude that CACT and IACT have the potential to reduce respiratory artifacts and improve quantitation in myocardial perfusion SPECT/CT. The performance of IACT was comparable to CACT with reduced radiation dose.